The present invention relates to a method of producing a broad-band signal applied to the end cap electrodes of an ion trap mass spectrometer for measuring in various analyzing modes.
An ion trap mass spectrometer is composed of a ring electrode having a hyperboloid-of-one-sheet-of-revolution internal surface and a pair of end cap electrodes having hyperboloid-of-two-sheets-of-revolution internal surfaces facing each other with the ring electrode therebetween. When a radio frequency AC (Alternating Current) voltage is applied between the ring electrode and the end cap electrodes, a quadrupole electric field is generated in the space surrounded by these electrodes (the space is hereinafter referred to as the xe2x80x9cion trap spacexe2x80x9d), in which ions produced within the ion trap space or ions introduced there from outside can be trapped.
In an ion trap mass spectrometer, various analyzing modes are possible by applying corresponding appropriate voltages to the end cap electrodes after ions are trapped in the ion trap space. When an AC voltage of a specific frequency fa, as shown in FIG. 6A, is applied to the end cap electrodes, only ions having a specific mass number (mass/charge) corresponding to the frequency fa resonate and oscillate, and are discharged from the ion trap space. If collision gas is introduced in the ion trap space and the voltage is properly applied, ions having a specific mass number are excited and fragmented. This method is used in an MS/MS analysis.
When an AC voltage signal containing frequency components ranging from fb to fc, as shown in FIG. 6B, is applied to the end cap electrodes, ions having several mass numbers corresponding to the frequency range fb-fc are simultaneously resonated and discharged from the ion trap space. When an AC voltage signal devoid of frequency components ranging from fd to fe (which is called a notched frequency range), as shown in FIG. 6C, is applied to the end cap electrodes, ions having mass number corresponding to the frequency rage fd-fe remain in the ion trap space and the other ions are discharged.
The voltage signal containing frequency components ranging from fb to fc as shown in FIG. 6B or that of notched frequency range as shown in FIG. 6C (which are hereinafter referred to as xe2x80x9cbroad-band signalxe2x80x9d) can be produced through a signal processing on a computer. In concrete, a broad-band signal can be produced by superposing (or adding) the component sinusoidal signals each having a singular frequency. If such component sinusoidal signals are simply superposed one by one, however, the amplitude of the resultant signal gradually increases and a large capacity power source is needed, which will increase the cost of such a device. Practically, therefore, when superposing the component sinusoidal signals, the initial phase angle of every sinusoidal signal is appropriately selected and shifted so that the amplitudes are cancelled with one another while the frequencies are preserved and properly incorporated in the resultant signal. The required dynamic range of the power source is thus suppressed.
One of conventional methods for finding such an appropriate initial phase angle of every component sinusoidal signal is as follows. When every sinusoidal signal is superposed to a temporary superposed signal, the initial phase angle of the sinusoidal signal is, starting from the 0xc2x0, shifted stepwisely by a certain amount (1xc2x0, for example), and the amplitude of the temporary superposed signal is examined to find out an appropriate initial phase angle that makes the amplitude minimum. This method is advantageous in assuredly finding the optimal initial phase angle for the minimum amplitude, but has a shortcoming in the enormous calculation that takes a long time even with a high-spec computer.
An improved method is described in the publication No. H07-509097 of Japanese translation of PCT international application as follows. The initial phase angle of every component sinusoidal signal is shifted by larger steps (10xc2x0, for example) to find out a rough optimal initial phase angle that brings about the nearest minimum amplitude, and the initial phase angle is shifted near the rough optimal initial phase angle by smaller steps (1xc2x0, for example) to find out an exactly optimal initial phase angle that makes the amplitude minimum. This method still requires a large amount of calculations, which is disadvantageous especially when the components of a frequency range are rearranged and a subsequent re-calculation is needed to find out new optimal initial phase angles of the component sinusoidal signals.
Further improvement to the method described above is known as follows. A hypothetical total signal containing all the frequency components possible and needed in designing a mass spectrometer is produced beforehand, where the optimal initial phase angles of all the component sinusoidal signals are found out and the component signals are added to produce the resultant total hypothetical signal. The hypothetical total signal and the data of the optimal initial phase angles of all the component sinusoidal signals are stored in a memory. When a signal containing a certain range of frequencies is required, component sinusoidal signals that are not contained in the range are excluded and subtracted from the total hypothetical signal based on the data of the optimal initial phase angles of the component signals.
It is true that the amplitude is minimum in the hypothetical total signal in the above-described method, but it is not necessarily true when some component signals are subtracted from the hypothetical total signal. There may be better initial phase angle or angles of the component sinusoidal signal or signals for producing a further reduced amplitude of a resultant signal.
Thus the conventional methods are not yet satisfactory in suppressing the dynamic range of a signal containing a range of frequency components. The present invention addresses the problem.
As explained above, there are two ways to approach the object of reducing the amplitude of a superposed signal: one is to add up component sinusoidal signals, and the other is to subtract unnecessary components from the total signal containing all the component sinusoidal signals. The former better assures the minimum amplitude of a resultant signal, but it requires a process of finding out optimal initial phase angles, where it takes a long time to generate a set of data of a sinusoidal signal and further enormous time to generate candidate sinusoidal signals having a variety of initial phase angles before the optimal initial phase angle is determined. The present inventor has studied the relation between the magnitude of the shifting step in finding out the optimal initial phase angle and the effect of suppressing the amplitude. The inventor has come to the following conclusion: the conventional knowledge that the smaller magnitude of the shifting step brings about the smaller amplitude of the superposed signal is not necessarily true. Instead, using either one of 0xc2x0 shift and 180xc2x0 shift of the initial phase angle for every component sinusoidal signal is sufficient, and there is no significant difference in the amplitude suppressing effect between the small step shifting approach and the two phase angle approach.
The inventor further devised a new method relating to the two phase angle approach. In place of adding the 180xc2x0 shifted component sinusoidal signal, the 0xc2x0 shifted component sinusoidal signal is subtracted. This method has a great advantage in that only 0xc2x0 shifted sinusoidal signal is needed for every frequency in superposing a component signal of the frequency and there is no need of generating many candidate sinusoidal signals of diverse initial phase angles.
Therefore, according to the present invention, the method of producing a broad-band signal including a plurality of component frequencies of regular or irregular intervals, where the broad-band signal is used to apply an alternating voltage to end cap electrodes of an ion trap mass spectrometer, includes the steps:
a) generating a sinusoidal signal having one of the component frequencies;
b) adding the sinusoidal signal to a current temporary superposed signal to produce an addition signal;
c) subtracting the sinusoidal signal from the current temporary superposed signal to produce a subtraction signal;
d) selecting either of the addition signal or the subtraction signal that has a smaller amplitude as a next temporary superposed signal; and
e) repeating the steps a)-d) for all the component frequencies.
Since there is no need to generate data of many candidate sinusoidal waves of different initial phase angles and no need to try many addition (superposition) of such candidate waves before finding a sinusoidal wave having an optimal initial phase angle according to the present invention, the processing time for producing a side range signal is greatly reduced compared to conventional methods. The effect of suppressing the resultant superposed signal is almost the same as conventional methods searching with smaller initial phase angle steps, so that the dynamic range of the voltage generator of an ion trap mass spectrometer can be made smaller and the cost can be reduced. This allows the user of the ion trap mass spectrometer to try various measurement conditions for the voltage applied to the end cap electrodes without worrying about the time needed to obtain he result.
The above-described invention is more easily understood by the description of a preferred embodiment referring to the attached drawings.